1. Field of the Invention
The present invention relates to a retrofocus type lens used for imaging lenses of cameras employing imaging devices such as CCD and image pickup tubes, silver halide film, or the like and for projection lenses for projector type televisions in general and, in particular, to a retrofocus type lens suitable for a projection lens of a projector type television employing a liquid crystal display panel.
2. Description of the Prior Art
When a long back focus is needed with a wide angle, there has conventionally been used a retrofocus type lens composed of a from group having a negative refractive power and a rear group having a positive refractive power. In such a retrofocus type lens, since distortion becomes greater as a wider angle is to be attained, an aspheric surface is used as a lens on the enlargement side so as to correct aberration.
Here, since the aspherical lens made of glass is more expensive, a plastic aspherical lens has been used in order to lower the manufacturing cost.
Plastic lenses, however, may be problematic in that, due to changes in temperature, their focal position tends to change greatly and their imaging property tends to deteriorate greatly. Also, materials usable for plastic lenses are more restricted than those of glass lenses in terms of refractive index and dispersion, they may be problematic in that it becomes further difficult to correct chromatic aberration in magnification which is hard to correct in a wide angle lens.